The present invention relates generally to synthetic aperture radar (SAR) and global positioning inertial navigation systems (GPS/INS), and more particularly, to a method of providing range measurements that uses the SAR and GPS/INS systems to measure the velocity of radar wave propagation at the same time that the range time lapse to a designated map pixel is measured, thereby improving measurement accuracy.
A synthetic aperture radar is able to provide a precise measurement of the round trip elapsed radar wave propagation time from an aircraft location to a target location. The accuracy of this range measurement is affected by the accuracy to which the velocity of radar wave propagation in the atmosphere is known. Atmospheric conditions affect the radar wave propagation velocity to 0.03 percent and more. Since the prime use of the synthetic aperture radar system is to penetrate cloudy weather under variable atmospheric conditions, the accuracy of the radar wave propagation velocity measurement is an important factor. In measuring range to the target using the synthetic aperture radar system, the round trip propagation time (T) of the radar wave is measured and the range is calculated as CT/2, where C is the radar wave propagation velocity. The value of C can vary as a function of altitude, humidity, clouds, and other attributes of weather. At 50 nautical miles, error due to the atmosphere can be 100 feet or more.
Therefore, in view of the above, it is an objective of the present invention to provide for a range measurement method for use with a synthetic aperture radar system that improves target location accuracy.